New to unraid- cache drive issue

I am running unraid 7 on a HP ProDesk 600 G3 SFF, there is one 4TB sata drive and a 256 nvme ssd.

I have made the 4TB drive my array with no mirror. And made the 256gb nvme as a drive in the cache pool.

My problem is that my cache drive keeps being reported as ‘unmountable - wrong or no file system’. I have tried formatting the drive with unraid a few times and rebuilding the cache pool but this makes no difference.